DESCRIPTION
This invention relates to a self-locking differential gear comprising:
a gear carrier whereon two crown wheels, each including a ring gear enmeshed with planet gears and an internally splined ring wheel with a respective axle shaft rotation-wise, are supported rotatably,
a friction clutch intervening between each crown wheel and said gear carrier to make the corresponding axle shaft unitary rotation-wise with said gear carrier, and
a device for engaging a corresponding clutch intervening between each clutch and the ring gear of the corresponding crown wheel, said device including first and second engagement members which are shiftable angularly relative to each other and respectively unitary rotation-wise with the ring rear and the corresponding axle shaft.
A differential gear of this kind is known from Patent GB-A-138,565.
In conventional self-locking differential gears, in particular for use on the front drive axle of four-wheel drive agricultural tractors, the crown wheels are mounted free to slide axially a short distance on the corresponding axle shaft to activate their respective clutches by compressing a corresponding set of friction plates each time that torque is transmitted from the differential gear to the axle shafts.
The axial sliding movement and consequent compression of the friction plates, which tends to drive the axle shaft at the same rotational speed as the gear carrier, is effected by the component of the thrust on the sides of the enmeshed teeth between the crown wheels and planet gears, along the axial direction of the axle shaft. In order for this axial thrust to develop in such a way as to successfully lock the differential gear, a sufficient resisting torque is to be applied to both axle shafts. This torque is due to the tractor resistance to motion and the resistance to rolling developed between the front drive wheels of the tractor and the ground on which they are acting.
However, when either of the two front drive wheels meets a condition of very poor traction, such as on mud, snow, ice, etc., it may occur that the resisting torque which opposes the driving torque induced on an axle shaft by the differential gear becomes so low as to apply an insufficient axial thrust to the sides of the enmeshed teeth between the crown wheels and the planet gears to produce an effective compression of the friction plates. In this case, that wheel which is under a condition of poor traction will begin to slip, thereby the differential gear locking action is lost.
A similar condition is likely to occur where one of the wheels, for example, is raised off the ground by upward bumps of the ground surface.
To partly obviate such shortcomings, UK Patent No. 138,565 suggests of increasing the clutch plate compression while running, with the provision of ramp devices intervening between the clutch and a ring gear on each crown wheel. With the proposed construction, the ramp devices produce an axial compression of the clutch plates, while torque is being transferred to the axle shaft, in either directions of rotation of the same and, therefore, either travel directions of a vehicle equipped with such a differential gear.
Thus, in essence, each time there occurs transfer of torque, whether driving or consisting torque, between an axle shaft and the corresponding crown wheel, the clutch plates will be equally compressed, thus tending to hold back each axle shaft with respect to the gear carrier.
In order for this not to hinder the differential splitting of the motion on turning, it is therefore necessary that the braking ability of the clutch plates or the axial thrust developed by the ramp devices be limited, and be limited, accordingly, the differential locking effectiveness. This drawback is enchanced where the differential is mounted on the front, driving and steering, axle of an FWD vehicle. Due, in fact, to the front wheels travelling, in the steered condition, a path with a larger average radius than that travelled by the rear wheels, they tend to slow down the vechicle with respect to the travel speed imposed by the rear wheels. This problem is obviated customarily by so splitting the transfer of motion as to have the tanqent velocity of the front wheels slightly higher, in straight-line running, than that of the rear wheels; however, as the steering angle is increased, that difference in velocity reduces itself down to negative values on tight cornering. In this case, the differential gear of UK Patent 138,565 would tend to become locked and hinder, if not altogether block, the steering action.